This invention relates to superconductive materials and, in particular, to the forming of superconductive ternary metal oxide materials and patterns from such materials.
The recent discovery that certain ternary metal oxide ("TMO") materials exhibit superconductivity at temperatures much higher than previously thought possible has aroused great interest, mainly because of its potential technological applications. Work to date has generally concentrated on the identification and growth of selected materials which possess the appropriate phase and stoichiometry, and on characterization of their transport properties, crystallography, and physical and chemical stability. More recently, the deposition of thin films of these materials has been reported.
Generally speaking, superconductivity is a low temperature phenomenon in which the affected material undergoes a transition to a state in which it exhibits the remarkable properties of negligible resistance to a flow of electrons, unusual magnetic effects, such as a large diamagnetism, altered thermal properties, including changes in its thermodynamic equilibrium and thermal transport properties, etc. The temperature corresponding to the midpoint of the temperature range over which the transition to superconductivity occurs is called the critical temperature, T.sub.c.
The observed critical temperatures of superconductors has slowly increased since the discovery of the superconductivity phenomenon three-quarters of a century ago, from just over 4.degree. K. back then to approximately 23.degree. K. in 1986 as new materials and techniques were explored. Many of these superconducting materials, if not most, were cubic niobium compounds. Since 1986 a new class of TMO superconductors, based upon copper oxides, has raised the T.sub.c limit to over 90.degree. K.
The ensuing international focus on this new class of superconducting materials is due to many reasons. First, these metal oxides are easier to fabricate than the highly refractory niobates. Second, their transition temperatures are sufficiently high to permit the use of more practical cryogenic media, such as liquid nitrogen and more cost-effective cryogenic systems. Third, these materials raise the prospect for superconductivity at room temperatures, i.e., at approximately 294.degree. K. And fourth, the benefits of a superconductive device employing the new materials may very well soon exceed the fabrication and operating costs which have hamstrung the commercial introduction of many superconducting devices.
Superconducting devices are expected to perform a wide range of functions, capitalizing on their unique characteristics. Such devices may be conveniently divided into two categories, small-scale electronic devices used in electronic instrumentation and computers, and large-scale devices typically having superconducting windings and used in high-energy physics research, nuclear magnetic resonance imaging (MRI), and power generation.
Commercial exploitation of TMO superconductors presently is hindered by the state-of-the-art in fabricating the materials themselves and patterning conductors made from the material.
The TMO superconducting materials are presently fabricated by mixing appropriate cations and subsequently heating the mixture for several (e.g. five) hours in an oven. Afterwards, the sample is ground and pressed into pellets, then sintered in an oven at 900.degree. C. to 1100.degree. C. X-ray diffraction has revealed that samples prepared in this way consist of several phases, and the relative proportion of these phases in a sample determines the superconductivity and depends on the heat treatment. Additional annealing is sometimes necessary to improve the superconducting properties.
The details of the preparation of the material determine not only the temperature of the transition to superconductivity, but also whether such a transition will occur for a given material. For example, the temperature extremes and duration, as well as the heating and cooling rates during sintering and annealing influence the oxygen concentration of the sample, which appears to be a key determinant in achieving high-T.sub.c superconductivity.
The superconductivity level of a sample can be determined by comparing the sample's magnetic susceptibility with that of an ideal superconductor. Alternatively, DC measurements of resistivity as a function of temperature can be used to gauge superconductivity. As the TMO films are presently fabricated, however, these tests are generally conducted and the quality of the sample as a superconductor determined after it is completely formed.
When fabricated in thin films for small-scale devices, the traditional superconducting materials, e.g., niobium compounds, have generally been patterned into superconducting circuits using conventional patterning techniques. Commonly used patterning techniques, however, are not appropriate for all materials. It appears that they may not be appropriate for the new TMO's. For example, many copper oxide-based TMO's are extremely sensitive to moisture. In the presence of water or water vapor, the trivalent copper ions are reduced to the bivalent state, accompanied by the irreversible decomposition of the material. Therefore, patterning y conventional lithographic methods which generally include exposure to water-containing chemicals appears to have series drawbacks for this material.
Accordingly, an object of the invention is to provide improved methods and systems for fabricating and patterning thin film superconductors.
A further object of the invention is to provide an improved superconductive device.
Yet a further object of the invention is to provide methods and systems for real-time, on-line monitoring of superconductive thin films during fabrication and patterning.